How to add load torque to the PMSM in Multisim model?

[redzzb]

I want to add load torque to my PMSM in the Multisim, and I did like this(Attached):

But the simulation results is not right. Why?

[BMac]

Great question.

See the Multisim help topic "Designing in the Mechanical Domain" for information on how to model mechanical inertias, torques and gearbox components. Summary:

• On the motor shaft terminal the mechanical speed is represented as voltage and the mechanical torque is represented as current.
• An ABM_CURRENT source is used to represent the mechanical torque from friction that increases with the shaft speed (or shaft speed squared or any other relationship.)

Here is a quote and screenshot:

"You  use electrical elements directly to model mechanical behavior. For instance, you can connect an independent voltage source to simulate an external force that turns the rotor at a speed equal to the voltage value. External loads may be modeled by attaching models to the rotor pin.

Notice that in the figure in Machine Modeling, a capacitor is used to represent rotor inertia. This is because the rotor inertia governs the relationship between the speed and torque in the same way that a capacitor governs the relationship between voltage and current. The same logic applies to the use of a resistor to represent the friction (with the exception that increasing friction tends to increase the amount of torque that is generated whereas increasing resistance tends to decrease the current, so we use 1/F_rotor as the resistance value).

The following is an idealized example of a coupled electromechanical system:

"The shaft of induction machine M1 is coupled to the shaft of DC machine M2, which acts as a generator, through ideal gears U3. Additionally, an external speed-dependent load is applied using a controlled current source I1. Because Multisim is programmed to interpret variables as voltages and currents, we must use the V(speed) function in I1 to reference what is actually the rotor speed. Notice the set of measurement pins on M1 and the one measurement pin on M2. You can probe the voltage on these pins to measure the stated physical quantity but you should not drive these pins - they act as outputs only. "

Here is where to find the Analog Behavioral Modeling Current Source (ABM_Current) on the components palette in Multisim. The component is described as follows: "Circuit variables (voltage, current, simulation time) along with various mathematical and logical functions and operators can be used to control this source's output":

Also be sure to check out the Mechanical Loads palette, which includes gears, inertial loads, and arbitrary load profiles.

And of course the Machines palette. Here's a screenshot for those who haven't browsed it yet: